Rydberg atoms and superconducting cavities are remarkable tools for the exploration of basic quantum phenomena and quantum information processing. These giant atoms are blessed with unique properties. They undergo a strong distance-dependent dipole-dipole interaction that gives rise to the dipole blockade mechanism : in the Van der Waals regime, this energy shift scales as n11, where n is the principal quantum number. If we shine an excitation laser tuned at the frequency of the isolated atomic transition on an atomic cloud, we expect to excite at most one atom within a blockade volume of (8 μm)3. We have set up an experiment to prepare deterministically one Rydberg atom. It uses a small cloud of ground-state Rubidium 87 atoms, magnetically trapped on a superconducting atom chip at 4 K, and laser-excited to the Rydberg states. The dipole blockade effect is sensitive to the line broadening due to the stray electric fields. Once an atom has been excited to our target state |60S1/2>, we explore the narrow millimeter-wave transitions between Rydberg states in order to assess these stray fields . With a gold-coated front surface for the chip, we observe as other groups large field gradients due to slowly deposited Rubidium atoms. We circumvent this problem by coating the chip with a metallic Rubidium layer. This way the gradients are reduced by an order of magnitude. This improvement allows us to observe extremely high coherence times, in the millisecond range, for Rydberg atoms near a superconducting atom-chip.
Theoretically, we present a simple scheme for the fast and efficient generation of quantum superpositions of two coherent fields with different classical amplitudes in a cavity. It relies on the simultaneous interaction of two two-level atoms with the field. Their final detection with a high probability in the proper state projects the field onto the desired mesoscopic field state superposition (MFSS). We show that the scheme is notably more efficient than those using a single atom. This work is done in the context of cavity QED, where the two-level systems are circular Rydberg atoms whose lifetime may reach milliseconds, interacting with the field of a superconducting microwave cavity. But this scheme is also highly relevant for the thriving field of circuit-QED. In both contexts, it may lead to interesting experimental studies of decoherence at the quantum-classical boundary.